Process for the manufacture of a multipolar elongate-electrode lens or mass filter

ABSTRACT

The method includes the steps of assembling one or more blanks (1) in supporting means (2, 3) so that the or each blank occupies at least the space to be occupied by elongate electrodes and, without disturbing the position of the blanks relative to said supporting means, removing material from all said blanks to generate said electrodes in position in said supporting means. Preferably the material is removed by an electrode-discharge machining (EDM) process, e.g. diesinking. The method avoids time-consuming alignment of preformed electrodes in said supporting means.

This invention comprises an improved method of manufacture of multipolarelongate electrode structures suitable for electrostatically focusing ormass-filtering a beam of charged particles. The method is particularlysuitable for the manufacture of a quadrupole mass filter.

Electrostatic lenses comprising a plurality of parallel elongateelectrodes are in common use for focusing and/or filtering beams ofcharged particles. Typically they comprise four or six rod electrodesdisposed parallel to and equidistant from an axis along which theparticles are travelling. The rods are insulated from one another sothat AC or DC potentials may be applied to them according to therequired purpose. The elongate electrodes may be cylindrical or may havea hyperbolic cross-section. Other geometries comprising more electrodes(e.g. 8 or 12), or a single rod electrode and a `V` angled electrode (a"monopole" mass filter) are also known, and may be manufacturedaccording to this method.

In all such lenses or filters the electrodes must comprise electricallyconductive material and means must be provided for holding them in placewhile providing electrical insulation between them. Particularly in thecase of quadrupole mass filters the electrodes must be precisely alignedto ensure high transmission efficiency at high mass resolution. Althoughseveral of the prior methods of construction described below areconventionally used to manufacture elongate electrode structures, thereremains a need for a lower cost method of production, particularly ofhigh performance quadrupole mass filters.

Most quadrupole mass filters are constructed from four accurately groundcylindrical or hyperbolic electrodes which are supported by two or moreceramic ring insulators comprising accurately formed locations for theelectrodes (see, for example, U.S. Pat. No. 4,032,782, SU patent868,885, GB patent 2,138,201 and JP patent application 58-204464).Manufacture of the ring insulators is however difficult and considerabletime is required to align the rods when the filter is assembled. Inorder to reduce the amount of precision machining of ceramic components,several prior designs incorporate metallic end-plates to which theelectrodes are attached through insulated washers, bushes or pins (forexample as disclosed in JP patent application 62-103956 and SU patent469479), or by specially shaped insulators (SU 989614, 989615). In otherforms of construction, electrodes made of an insulating material coatedwith a conductive film may be fitted to metallic endplates, as disclosedin U.S. Pat. Nos. 3,699,330 and 3,793,063. U.S. Pat. No. 4,870,283discloses metallic electrodes mounted in locations machined in ametallic yoke but spaced from the yoke by a thin insulating film.Further methods comprise spacing the electrodes apart by saphire ballswhich locate in dimples formed in the electrodes (Munro, Rev. Sci.Instrum, 1967, vol. 38 (10) pp 1532), spacing the electrodes apart byprecision machined cylindrical insulators disposed around the inside ofa cylinder (Okayama, Nucl. Instrum. Methods in Phys. Res. 1990 vol. A298pp 488-495), and the use of ceramic ring insulators without locations inconjunction with non-circular electrodes which have an outwardly facingradius of curvature equal to the inside radius of the ring insulator(U.S. Pat. No. 3,553,451). A ceramic ring insulator without locations isalso used in the structure disclosed in SU 694917, wherein a metallicend-plate for locating all the electrodes is brazed inside a ringinsulator and subsequently cut into sections (typically by sparkerosion) to provide electrical insulation between the electrodes. Inanother variation using ceramic ring insulators (U.S. Pat. No.3,840,742) a method of locating and fixing hyperbolic electrodes isdisclosed.

A different approach is to form the structure of the lens or mass filterfrom an insulator and provide the conductive electrodes by coating aconductive material on the structure as required. GB patent 1,367,638discloses a quadrupole filter formed in a ceramic cylinder comprising anaxial passage having four hyperbolic surfaces which are gold plated tocreate the electrodes. Alternatively, the body can be formed from athermally softenable insulator such as glass or quartz which can bemoulded on a mandrel. Quadrupole mass filters made in this way aredisclosed in U.S. Pat. Nos. 3,328,146 and 4,213,557, German patentapplication 1,297,360, and European application 268,048. U.S. Pat. No.4,117,321 discloses a quadrupole filter comprising metallic electrodesmounted in a body made from a thermally mouldable insulator previouslyformed on a mandrel. U.S. Pat. No. 4,106,744 discloses another variationwherein eight elongate rectangular cross-section insulators are securedby clips on to a mandrel and a layer of metal is deposited over theentire assembly to form a unitary structure from which the mandrel isthen removed. The mandrel comprises hyperbolic or circular surfaces onwhich the deposited metal creates electrodes of the desired form. Afterthe mandrel has been removed, the deposited metal which overlays theinsulators at the extremities of the electrodes is removed so that theelectrodes are electrically insulated from each other.

Up to now, the great majority of commercially available quadrupole massfilters employ the ceramic ring/metal electrode structure. The solidinsulating body/plated electrode structure is available commercially asa relatively low performance mass filter. There is a need, therefore,for a method of manufacturing multipolar elongate electrode structures,particularly those capable of operation as a high performance quadrupolemass filter, more cheaply than is possible with the conventional ceramicring approach. It is an object of the invention to provide such amethod. It is another object to provide a method which avoids the timeconsuming process of electrode alignment inherent in many of the priormethods of manufacturing multi-polar electrode structures.

The invention provides a method of manufacturing a multipolar electrodestructure for focusing or mass-filtering a beam of charged particles,wherein said structure comprises a plurality of elongate electrodesdisposed substantially parallel to an axis, said method comprising:

a) providing a sufficient number of blanks of electrically conductivematerial to permit the manufacture of all of said elongate electrodes,each of said blanks comprising enough material for at least one saidelectrode;

b) providing insulating supporting means into which all of said blankscan be mounted and which is capable of maintaining said electrodes infixed spatial relationship to each other after completion of steps c)and d);

c) assembling all said blanks and said supporting means so that saidblanks occupy at least the space to be occupied by said electrodes; and

d) without disturbing the position of said blanks relative to saidsupporting means removing material from all said blanks to generate saidelectrodes in position in said supporting means.

In contrast with the prior structures comprising electrodes made fromelectrically conductive material, the method of the invention providesfor the generation of the desired profile of the electrodes after theblank(s) from which they are to be made are fitted to the supportingmeans, thereby eliminating the time-consuming alignment processnecessary with preformed electrodes and reducing the cost of manufactureof the completed structure. Conveniently, the step of removing materialfrom the blanks to generate the electrodes is either a wire-cutting ordiesinking electro-discharge machining process (EDM) o The blank(s) maycomprise a single piece of material from which all the electrodes may becut or separate pieces of material from which one or some of theelectrodes are cut. The electrodes may be generated with any desiredform, but typically a circular or hyperbolic profile is used. The methodcan generate hyperbolic profiles with the same ease as circularprofiles, in contrast with most prior methods, and provides anespecially convenient way of manufacturing a hyperbolic quadrupole massfilter.

The supporting means may comprise a single insulating member butpreferably two such members are provided, spaced apart from one anothertowards the ends of the elongate electrodes, that is, disposed in asimilar place to that occupied by the ceramic ring insulators in aconventional quadrupole filter assembly employing accurately machinedelectrodes. The insulating members may comprise ring insulators, but incontrast to the conventional type these do not have to be accuratelymachined. It is necessary only that they provide means for securing theelectrode blank or blanks so that the completed structure will remain inproper alignment. Thus the blanks (typically stainless steel ormolybdenum, although conductive ceramic or aluminium may also be used)may be brazed into ceramic supporting rings or they may be secured byscrews as in many prior devices.

In one preferred method the insulating members may comprise annularceramic insulators and the electrode blank may comprise a solid cylinderof stainless steel, molybdenum, conductive ceramic or aluminium oflength equivalent to the desired length of the completed electrodes anda diameter such that it is a good fit inside the ceramic insulators. Thecylinder is first secured into the insulators (either by brazing or byscrews located so that the completed electrodes will be held in positionafter machining is completed). An axial hole may then be drilled in theblank (through material which will subsequently be removed) and theassembly set up on a numerically controlled wire-cuttingelectro-discharge machine (EDM) with the wire passing through the hole.The EDM is used to cut the blank to leave the desired number ofaccurately formed separated electrodes attached to the insulators.Alternatively, several blanks may be roughly machined and fitted to theinsulating members in place of the single cylinder, and then machined bythe EDM to produce the desired electrode structure. Obviously, each ofthe blanks must contain sufficient material for at least one electrode.

When it is desired to produce an electrode structure which comprisesseveral sections having similar electrode structures in alignment witheach other, for example a quadrupole mass filter with a prefiltercomprising several short electrodes aligned with the main electrodes ofthe filter but electrically isolated from them, a preferred methodcomprises the use of blanks which extend the entire length of theelectrode structure. After machining the blanks to the desired profile,the EDM may then be used to cut the electrodes into segments asrequired. In such a case the supporting means must be such that thesegmented structure is properly supported after the electrodes have beencut.

In a case where the electrodes are very long, the wire cutting EDM mayproduce an electrode structure wherein the distance from the axis to theelectrodes measured at the centre of the structure is slightly greaterthan that measured at the ends, due to the vibration of the wire duringthe cutting process which has its maximum amplitude at the centre of thewire. In such a case the wire-cutting EDM may be programmed to produceelectrodes which are slightly oversize, and the machining may then becompleted on a diesinking EDM using an electrode having the desiredprofile of the internal space inside the electrode structure. In thediesinking process, this electrode is slowly advanced along the axis ofthe electrode structure causing a shape complementary to that of theelectrode to be imparted to the electrode blanks.

When diesinking EDM is used to form the electrode structure the cuttingprocess used to machine the blank(s) to produce oversize electrodesprior to the diesinking process is not limited to wire-cutting EDM.Oversize electrodes may also be formed by processes such as moulding,e.g. casting, or extrusion.

If necessary, the surface finish of the electrode structure produced byEDM may be improved by a conventional polishing process, for example,electropolishing.

Preferred methods according to the invention will now be described indetail by way of example only and by reference to the figures, in which:

FIG. 1 is a drawing of an electrode blank and two supporting membersassembled prior to electrodischarge machining;

FIG. 2 illustrates how the assembly of FIG. 1 may be machined by awire-cutting EDM to generate the electrodes;

FIG. 3 shows a completed electrode structure after electrodischargemachining;

FIG. 4 is a drawing-of an electrode for a diesinking electrodischargemachine suitable for any necessary final machining of the electrodestructure of FIG. 3; and

FIG. 5 is a cross-sectional view of a quadrupole mass filtermanufactured according to the method of the invention.

According to the invention an electrode structure suitable for use as aquadrupole mass filter may be manufactured from an electrode blank 1ccmprising a solid cylinder of e.g. stainless steel, molybdenum,conductive ceramic or aluminium which is at least as long as theelectrodes of the completed structure. The blank is selected (or ifnecessary, machined) to be a good fit in two insulating members 2, 3which comprise the insulating supporting means. Insulating members 2, 3comprise short ceramic cylinders with central apertures 4, 5. Theelectrode blank 1 is attached to each of the insulating members 2, 3 byfour radially disposed screws 6, 7 spaced at 90° intervals around thecircumference of the insulating members. The screws 6, 7 engage tappedholes in the electrode blank 1, and flats 8, 9 may be provided on theinsulating members 2, 3 underneath their heads. The flats 8, 9 help toprevent the screws 6, 7 working loose and also provide convenientsurfaces for mounting the completed structure in the vacuum envelope ofa mass spectrometer, etc. The positions of the insulating members 2, 3on the blank 1, and the location of the screws 6, 7 are selected so thatthe completed electrodes will be firmly held in the insulating membersafter machining. It will be appreciated that the supporting means mayalternatively comprise a single insulating member, or more than two suchmembers, according to the type of structure required.

As an alternative to the use of the screws 6, 7, the blank 1 may bebrazed or soldered to the insulating members 2, 3. It is also possibleto use more than one blank, providing that each blank comprises enoughmaterial for at least one electrode and all the blanks can be fittedsimultaneously into the insulating members. For example, four blankscorresponding to the quadrants into which the blank 1 is divided by thedotted lines 10 (FIG. 1) may be used. It is not necessary for either theblank(s) or the insulating members to be manufactured within veryprecise tolerences, as would be the case for insulating members andelectrodes of a conventionally constructed quadrupole mass filter. Forexample, suitable blanks can be made by moulding, casting or extrusion.

If a wire-cutting EDM is to be used, it is necessary to provide a holeor slot in the blank 1 through which the wire of the EDM can be threadedprior to starting the machining. This may comprise an axial hole 11 madeby any suitable process (e.g. drilling or boring by another EDMprocess), or it may comprise a radial slot 12 cut along the length ofthe blank. Clearly the hole 11 or the slot 12 must be made in a portionof the blank which will be removed during machining. If more than oneblank is used, this step may be omitted and the wire simply threadedbetween the blanks.

The assembly of FIG. 1 is then clamped to the work table 13 of awire-cutting electrodischarge machine as shown in FIG. 2, with the EDMwire 14 running through the hole 11 (or the slot 12) as indicated. Asuitable machine for manufacturing a 150 mm long quadrupole is the theFANUC W2 wire-cutting EDM, but many other suitable machines areavailable commercially. The EDM should comprise means for moving thework table 13 accurately to any desired position in the x-y plane underthe control of a digital computer in order to facilitate the accurategeneration of the desired electrode profile. The electrodischarge (i.e.,spark erosion) is carried out by application of a high frequency pulsedDC supply (illustrated schematically at 15) which is applied between theEDM wire 14 and the electrode blank 1. This results in small sparkspassing between the wire 14 and the blank 1 which remove material fromboth the blank and the wire. Thus, by slowly moving the blank relativeto the wire a cut of extreme precision can be made with a width onlyvery slightly greater than the diameter of the wire. With suitableprogramming the computer can move the blank to cut the electrode profileto any desired shape. On modern machines it is possible to use theoutput of a computer-aided-design (CAD) program (which may be used todesign the electrode structure) to produce machine readable instructionsfor the EDM which will program the machining process without humanintervention. It will be seen therefore that the EDM can generateelectrode surfaces having either hyperbolic or circular surfaces withequal ease, in marked contrast to those prior methods of constructioninvolving machining the electrodes separately.

In order to avoid breakage of the wire through erosion, the EDM providesmeans for driving the wire from a supply reel, over a drive pulley 16,through the workpiece, a second drive pulley 17 and into a take-upchamber so that the cutting is carried out with uneroded wire throughoutthe entire process. Means are also provided for providing tension in thewire between the pulleys 16 and 17. A liquid electrolyte, typicallywater with additives to reduce its corrosive properties and control itsconductivity, is pumped through one or more nozzles 18 through theworkpiece in the vicinity of the wire 14 both to remove eroded materialand to provide the most suitable environment for the spark erosion.Table 1 summarizes the machining conditions which are thought to besuitable for the manufacture of a typical quadrupole mass filter 150 mmlong on a FANUC W2 wire-cutting EDM.

                  TABLE 1                                                         ______________________________________                                        Parameter         Value                                                       ______________________________________                                        discharge voltage 85 V                                                        discharge on time 8 μs                                                     discharge off time                                                                              8 μs                                                     electrolyte       water, resistivity                                                            1.5 × 10.sup.4 Ω · cm                  wire              brass,                                                                        0.25-0.3 mm diameter                                        wire tension      1000-1200 g                                                 cutting speed     ≈0.2 mm/min                                         ______________________________________                                    

FIG. 3 shows the completed electrode structure after the wire-cuttingEDM is completed. It comprises four electrodes 18-21 which are attachedto the insulating members 2 and 3 by the screws 6, 7 and requires nofurther alignment because the electrodes have been accurately profiledrelative to each other by the discharge machining. It is necessary onlyto degrease and clean the assembly to complete the manufacturingprocess.

If a highly polished finish is needed on the electrodes 18-21, theassembly of FIG. 3 may be electropolished in a conventional way. Thisinvolves the immersion of the assembly in a suitable polishing bath andmaintaining an electrical current between the electrodes 18 and 21 andanother electrode in the bath. A small amount of material removed fromthe electrodes leaving a very highly polished finish. Electropolishingbaths suitable for stainless steel or molybdenum are well known andavailable commercially.

The most preferred method of completing the electrodes is diesinkingEDM. A blank comprising oversize electrodes is first manufactured by anysuitable method (e.g., moutding, casting, extrusion or the wire-cuttingEDM process described above), and the machining is completed bydiesinking EDM using an electrode of the type shown in FIG. 4. The dieelectrode 22, typically copper, may be manufactured by a wire-cuttingEDM to have a profile equivalent to the inner space 23 (FIG. 3) of thecompleted electrode structure but very slightly undersize. Typically theelectrode 22 is 5-10 mm in depth and can therefore be manufacturedwithout any significant concavity. Die 22 is then fitted to a diesinkingEDM (for example, a Bohrmeister 280) and the assembly of FIG. 3positioned on its work table so that the die 22 can be advanced slowlyalong the axis of the structure, causing the electrodes 18-21 to befurther machined with a shape complementary to the die 22 and with agreater uniformity than is possible with wire-cutting alone. Diesinkingmachines operate at much higher discharge currents than do wire-cuttingmachines and this results in wear of the die electrode as it is used.However, it is estimated that about 10 electrode structures of the typeshown in FIG. 3 could be machined with one die electrode 22.

FIG. 5 shows a cross section of a quadrupole mass filter with hyperbolicelectrodes manufactured according to the invention. In order to machinethe electrode structure, the EDM is programmed to cut the electrodesurfaces 24 according to the equation: ##EQU1## and the electrodesurfaces 25 according to the equation: ##EQU2## where x and y are thecoordinates along the x and y axis shown in the figure and r₀ is theinternal radius of the electrode structure.

It will be appreciated that manufacture of a hyperbolic electrodestructure similar to that of FIG. 5 by a conventional method usingseparately machined electrodes located in accurately formed insulatingmembers is impractical because of the difficulty of making electrodeswith the necessary cross section and of subsequently aligning them inthe insulating members. The structure of FIG. 5 is particularlyadvantageous because it allows a filter with high r₀ to be constructedwithin a small overall diameter, resulting in a completed filter whichis much smaller than a conventionally constructed filter of similarperformance. The circles 26 in FIG. 5 illustrate how electrodes ofcircular cross-section would have to be mounted in insulating members 2and 3 of the same size, and clearly show that the r₀ of a filterconstructed in this way is very much smaller than that of the hyperbolicfilter manufactured according to the invention.

We claim:
 1. A method of manufacturing a multipolar electrode structurefor focusing or mass-filtering a beam of charged particles, wherein saidstructure comprises a plurality of elongate electrodes disposedsubstantially parallel to an axis, said method comprising:a) providing asufficient number of blanks of electrically conductive material topermit the manufacture of all of said elongate electrodes, each of saidblanks comprising enough material for at least one said electrode; b)providing insulating supporting means into which all of said blanks canbe mounted and which is capable of maintaining said electrodes in fixedspatial relationship to each other after completion of steps c) and d);c) assembling all said blanks and said supporting means so that saidblanks occupy at least the space to be occupied by said electrodes; andd) without disturbing the position of said blanks relative to saidsupporting means removing material from all said blanks to generate saidelectrodes in position in said supporting means.
 2. A method as claimedin claim 1 wherein the step of removing material from said blankscomprises an electro-discharge machining process.
 3. A method as claimedin claim 2 wherein said electro-discharge machining process is adiesinking process.
 4. A method as claimed in claim 3 wherein said blankor blanks comprise(s) oversized electrodes.
 5. A method as claimed inclaim 4 wherein said oversize electrodes are produced by wire-cuttingelectro-discharge machining.
 6. A method as claimed in claim 4 whereinsaid oversize electrodes are produced by moulding or extrusion.
 7. Amethod as claimed in claim 2 wherein said electro-discharge machiningprocess is a wire-cutting process.
 8. A method as claimed in claim 1wherein said supporting means comprises two insulating members spacedapart from one another towards the ends of said elongate electrodes. 9.A method as claimed in claim 8 wherein said insulating members compriseannular ceramic insulators and said electrode blank(s) comprisestainless steel, molybdenum, conductive ceramic or aluminium.
 10. Amethod as claimed in claim 1 wherein said electrode structure is aquadrupole mass filter.
 11. A method as claimed in claim 1 wherein saidelongate electrodes each comprise two or more electrode segments alignedwith one another, the method comprising the further step of subsequentlycutting the machined elongate electrodes to separate them into saidelectrode segments.
 12. A method as claimed in claim 1 wherein theelectrode structure is subsequently electropolished.
 13. A method asclaimed in claim 8 wherein said electrode structure is a quadrupole massfilter.
 14. A method as claimed in claim 9 wherein said electrodestructure is a quadrupole mass filter.
 15. A method as claimed in claim8 wherein said elongate electrodes each comprise two or more electrodesegments aligned with one another, the method comprising the furtherstep of subsequently cutting the machined elongate electrodes toseparate them into said electrode segments.
 16. A method as claimed inclaim 9 wherein said elongate electrodes each comprise two or moreelectrode segments aligned with one another, the method comprising thefurther step of subsequently cutting the machined elongate electrodes toseparate them into said electrode segments.
 17. A method as claimed inclaim 8 wherein the electrode structure is subsequently electropolished.18. A method as claimed in claim 9 wherein the electrode structure issubsequently electropolished.
 19. A method as claimed in claim 2 whereinthe electrode structure is subsequently electropolished.
 20. Anelectrode structure manufactured according to claim 1.